This invention relates to semiconductor junction lasers and, more particularly, to lateral current confinement (LCC) in stripe geometry junction lasers.
The stripe geometry junction laser was first proposed by R. A. Furnanage and D. K. Wilson (U.S. Pat. No. 3,363,195 issued on Jan. 9, 1968) as a means to reduce the number of lasing modes. The stripe geometry also reduces the threshold current for lasing, which alleviates heat sinking and other problems, and limits the spatial width of the output beam, which facilitates coupling into an optical fiber. Since that early proposal, numerous laser configurations have been devised to implement the stripe geometry concept, but clearly the front runner, both in terms of widespread usage as well as reliability, is the proton bombarded double heterostructure (DH) laser described by J. C. Dyment et al, Applied Physics Letters, Vol. 10, page 84 (1967), and L. A. D'Asaro et al, U.S. Pat. No 3,824,133 issued on July 16, 1974.
Notwithstanding the success of DH stripe geometry junction lasers delineated by proton bombardment, workers in the art have suggested a virtual plethora of alternative structures aimed primarily at one or more objects such as lowering the lasing threshold, controlling filamentary light outputs and producing more symmetric light beams. One such configuration is the LCC junction laser in which the stripe or channel through which current flows under forward bias to the active region is delineated by laterally separated reverse-biased p-n junctions. The space between the junctions defines the stripe.
One type of LCC stripe geometry junction laser is the heteroisolation laser described by K. Itoh et al in IEEE J. Quant. Electr., Vol. QE-11, No. 7, pp. 421-426 (1975). This laser is a conventional n-n-p AlGaAs DH except that a layer of n-AlGaAs is grown on the p-GaAs cap layer to form a blocking junction at the interface therebetween. A stripe is etched through the n-AlGaAs layer so as to expose the underlying p-GaAs cap, thereby bifurcating both the n-AlGaAs layer and the blocking junction. A metal contact is then deposited over both the bifurcated n-AlGaAs layer and the exposed p-GaAs stripe. When the active region p-n junction is forward biased, the bifurcated p-n junction is reverse biased. Thus, current through the n-AlGaAs segments is blocked and constrained to flow through the p-GaAs stripe to the active region. Thresholds in the Itoh et al lasers were, however, relatively high; for example, 460 mA for the fundamental mode at 7620 angstroms (FIG. 5) and 3000 A/cm.sup.2 for an active region thickness of 0.2-0.3 .mu.m (FIG. 4).
Another variant of the LCC stripe geometry junction laser, of the type which utilizes reverse biased p-n junctions to delineate the stripe, is taught by R. D. Burnham et al in IEEE J. Quant. Electr. Vol. QE-11, No. 7, pp. 418-420 (1975). A stripe mask is deposited on an n-GaAs substrate and then Zn is diffused into the exposed portions. Laterally separate blocking p-n junctions are thus formed in the substrate. Then, a conventional n-p-p AlGaAs-GaAs-AlGaAs DH is grown on the diffused substrate surface. When the active region p-n junction is forward biased, the blocking junctions in the substrate are reverse biased thereby constraining current to flow through the stripe therebetween. As with the Itoh et al LCC lasers, however, thresholds were high. Pulsed thresholds were greater than about 150 mA (8000 A/cm.sup.2) for a stripe width of 10 .mu.m and an active region thickness of 0.45 .mu.m.
A further modification of their LCC laser is suggested by R. D. Burnham et al in U.S. Pat. No. 3,984,262 issued on Oct. 5, 1976 which describes the use of laterally separated reverse biased p-n junctions not only in the substrate but also in the top surface of the DH (col. 5, lines 1-19). Operating parameters, such as lasing threshold, are not given for the modified LCC laser.
What is apparent, however, is that the lasing threshold of this class of LCC stripe geometry DH lasers has fallen far short of an improvement over conventional DH lasers which routinely have thresholds of about 100 mA.